1. Field of the Invention
The present invention relates to a directional coupler type optical function element of a novel construction, and more particularly, to a directional coupler type optical function element having very high extinction ratio characteristics and adapted for use as an optical switch, polarizing splitter, optical modulator, wavelength division multiplexer/demultiplexer.
2. Description of the Related Art
Recently, various optical function elements having a directional coupler of a waveguide type have been developed, and optical switches, polarizing splitters, optical modulators, wavelength division multiplexer/demultiplexer, etc. using these elements have been proposed.
FIGS. 1 and 2 show examples of conventional optical function elements of a directional coupler type. The element shown in FIG. 1 is a 2-input/2-output element, while the element shown in FIG. 2 is a 1-input/2-output element.
In FIG. 1, a junction C.sub.0 of a length L is formed by arranging two optical waveguides A and B of equal widths W close to each other in parallel relation, with a distance G for evanescent connection between them.
Curved optical waveguides D.sub.1, D.sub.2, D.sub.3 and D.sub.4 with a path width W and curvature radius R are optically connected to the respective incidence ends A.sub.1 and B.sub.1 and emergence ends A.sub.2 and B.sub.2 of the optical waveguides A and B of the junction C.sub.0, respectively, thus forming incidence-side lead section C.sub.1 and emergence-side lead section C.sub.2. Also, straight optical waveguides E.sub.1, E.sub.2, E.sub.3 and E.sub.4 with the path width W are optically connected to the curved optical waveguides D.sub.1, D.sub.2, D.sub.3 and D.sub.4, respectively, with a distance G.sub.0 between the respective path-width centers of the waveguides E.sub.1 and E.sub.2 and between those of the waveguides E.sub.3 and E.sub.4. Electrodes F.sub.1, F.sub.2, F.sub.3 and F.sub.4 are mounted on the optical waveguides A and B of the junction C.sub.0 so that electrical signals can be introduced from the electrodes into the waveguides. The distances between the electrodes F.sub.1 and F.sub.3 and between the electrodes F.sub.2 and F.sub.4 are substantially zero.
If the straight optical waveguide E.sub.1 is an incidence port, the straight optical waveguides E.sub.3 and E.sub.4 serve as a through port and a cross port, respectively.
The 1-input/2-output element of FIG. 2 is a modified version of the 2-input/2-output element of FIG. 1, in which one straight optical waveguide E.sub.0 is optically connected to only the incidence end A.sub.1 of the optical waveguide A in a direct manner. In this element, the straight optical waveguides E.sub.3 and E.sub.4 serve as a through port and a cross port, respectively.
In order to incorporate these elements in a fiber communication system, which is going to be practically used, it is necessary to prevent errors attributable to cross talk. Thus, the elements are expected to be subject to less cross talk, that is, to have high extinction ratio characteristics.
In the case of the element shown in FIG. 1, a theoretically perfect cross mode is established to heighten the extinction ratio without limitation by applying proper electrical signals from the electrodes F.sub.1, F.sub.2, F.sub.3 and F.sub.4. Connections at the incidence- and emergence-side lead sections C.sub.1 and C.sub.2 cannot, however, provide a perfect through mode, and in this case, the theoretical value of the extinction ratio ranges from about 20 to 30 dB at the highest.
In the case of the element shown in FIG. 2, moreover, the extinction ratio for the through mode can be made about 10 dB higher than that of the element shown in FIG. 1. For the cross mode, however, the extinction ratio ranges from only about 10 to 20 dB.
Thus, the conventional elements, which have a low extinction ratio for the through or cross mode, cannot exhibit high extinction ratio characteristics for both the through and cross modes.
The extinction ratio used here is a value given by the followed equation: 10 log.sub.10 (.vertline.r.vertline..sup.2 /.vertline.s.vertline..sup.2), where .vertline.r.vertline..sup.2 is the output power of the through port, and .vertline.s.vertline..sup.2 is the output power of the cross port.
Among optical function elements constructed in this manner, known examples of those which have relatively high extinction ratio characteristics include an optical switch with an extinction ratio of about 27 dB reported in Technical Digest Integrated and Guide-wave Optics '86 by P. Granestrand et al. and a polarizing splitter with an extinction ratio of about 28 db reported in the 1990 Autumn National Meeting C-216 of the Institute of Electronic Intelligence and Communication Engineers of Japan by H.M. Mak et al.
Meanwhile, those optical function elements which are practically used in an optical communication system are expected to have an extinction ratio of 15 dB or more.
To meet this requirement, intense studies have been made of the causes of low extinction ratios of directional coupler type optical function elements.
Among these studies, there is one whose results are described in Institution of Electrical and Electronics Engineers Journal (IEEE. J.) of Quantum Electronics (Vol. 24, March, 1988) by Jean-Pierre Weber et al. This description indicates that the low extinction ratio is attributable to difficulty in refractive index control of the directional coupler for a required switching state.
L. McCanghan et al. also reported in in IEEE. J. of Quantum Electronic (Vol. QE-22, No. 6, June, 1986) that the low extinction ratio is attributable to the irregularity of the refractive index of optical waveguides with respect to the extending direction thereof (direction of light propagation).
Further, T. K. Findakly et al. indicated the following in Journal of Lightwave Technology (Vol. 6, No. 1, January, 1988). According to this report, the extinction ratio is inevitably lowered because switching state cannot be obtained in a through mode due to the existence of small connections at incidence- and emergence-side lead sections for connecting optical fibers.
Among the three causes described in these reports, the one reported by Jean-Pierre Weber et al. can be removed by properly selecting the method of driving the element and the material of the optical waveguides. Further, the cause reported by L. McCanghan et al. can be removed by improving film growth control during the formation of the optical waveguides.
The cause indicated by T. K. Findakly et al., however, is an unavoidable problem which cannot be solved unless the outside diameter of the optical fiber to be connected is reduced to several micrometers so that the element requires no lead sections.
In the case of the optical function element of the conventional construction shown in FIG. 1, the extinction ratio is lowered only for the through mode due to the connections at the incidence- and emergence-side lead sections C.sub.1 and C.sub.2 even though the element is a perfectly symmetrical directional coupler.
The extinction ratio characteristics of the optical function element depend on the lower one of the extinction ratios for the through and cross modes. Even in case the extinction ratio for the cross mode is unlimited, therefore, the whole element can enjoy only a low extinction ratio if that for the through mode is low.
In the case of the optical function element shown in FIG. 1, moreover, the extinction ratio for the cross mode can be also lowered if the incidence- and emergence-side lead sections C.sub.1 and C.sub.2 do not have the same configuration.
In many of practical versions of the element of FIG. 1, for example, the radius of curvature of the incidence-side lead section C.sub.1 is different from that of the emergence-side lead section C.sub.2, so that these sections are not symmetrical. In such a case, the extinction ratio for the cross mode, as well as that for the through mode, is lowered for the aforesaid reason.
Thus, in the conventional directional coupler type optical function elements, the extinction ratio for the through mode is low, and that for the cross mode is also lowered if the incidence- and emergence-side lead sections C.sub.1 and C.sub.2 are not symmetrical.